This invention concerns a semiconductor manufacturing technique and, more in particular, it relates to a technique effective to application for the improvement of the cleaning effect for cleaning inside a molding die for use in semiconductor devices, as well as improvement for the productivity.
The technique described below has been studied by the present inventors upon study and completion of the invention and the outline is as described below.
In the resin encapsulating step for resin encapsulation type semiconductor devices, since resin molding steps are repeated again and again, contaminants such as resin burrs, oxide films, oils or dusts are accumulated to an inside of a molding die in which an encapsulating resin is filled, that is, in the cavities and the runners, as well as the periphery of air vents and cull blocks of an upper die and a lower die forming a pair of molding dies.
Since such contaminants give undesired effects on the quality of molding and lower the releasability upon releasing of products from the molding die, an operator has to clean the molding die on every predetermined cycles of resin shots.
However, since cleaning for the molding die by the operator, being conducted by manual operation, requires a considerable period of time, a technique capable of cleaning the molding die in a short period of time has been demanded.
For coping with such a demand, Japanese Patent Laid-Open Hei 1(1989)-95010 discloses a cleaning method of clamping a lead frame not mounted with a semiconductor chip (hereinafter referred to as a dummy lead frame) between main surfaces (mating surfaces) of a molding die and injecting and hardening a cleaning resin formed, for example, of a melamine resin in the molding die, thereby depositing contaminants on the surface of the cleaning resin and removing the contaminants together with the cleaning resin.
Further, there is also a method of directly flowing a cleaning resin at a high pressure and a normal pressure into the cavity without using the dummy frame.
However, since an expensive dummy lead frame is used for cleaning in this technique, it is not economical, and a high accuracy is necessary for positioning the molding die and the dummy lead frame since the dummy lead frame of a predetermined shape adaptable to the molding die has to be set and clamped at a predetermined position of the molding die. Further, the cleaning resin formed in the cull portions or the runner portions are separated being detached from the lead frame and it requires a considerable time to remove the separated resin from the molding die to worsen the operation efficiency. Further, the separated culls and runners are put between the sliding portions of the molding apparatus to sometimes result in disorder.
In view of the above, for overcoming such problems, the techniques to be described below have also been proposed.
Japanese Patent Laid-Open Hei 6(1994)-254866 discloses a method comprising the steps of clamping a sheet-like material made of a cotton fabric (non-woven fabric) capable of impregnating and permeating a cleaning resin between opened molds, and filling a cleaning resin in a molten state into the cavity of closed molding dies.
As has been described above in the known literature, since a liquid cleaning resin is injected in a state where a sheet capable of impregnating and permeating a cleaning resin and a chemical is put between the main surfaces (mating surfaces) of upper and lower dies, the positioning accuracy required between the molding die and sheet can be lowered and the cleaning resin and the chemical penetrate also to portions where the sheet is put between the main surface of the upper and lower dies, thereby to conduct cleaning for the molding dies.